Pixels, Centimetres, Inches and DPI: Converting a Form's Photo Size
A form says 3.5 x 4.5 cm. Your tool asks for pixels. This is the one formula that connects them.
The three units, in one sentence each
Pixels are the tiny squares an image is made of. Centimetres and inches are physical lengths on paper or screen. DPI (dots per inch) says how many pixels are packed into each inch when the image is printed. A digital file has no physical size until someone chooses a DPI, which is why the same photo can be "3.5 cm wide" or "10 cm wide" depending on how it is printed.
The formula
To convert a physical size to pixels:
or, if the form gives inches: pixels = inches x DPI
Worked example: a form wants 3.5 x 4.5 cm at 300 DPI. Width: 3.5 / 2.54 = 1.378 inches, times 300 = 413 pixels. Height: 4.5 / 2.54 = 1.772 inches, times 300 = 531 pixels. So the file should be 413 x 531 pixels.
To go the other way, divide pixels by DPI to get inches, then multiply by 2.54 for centimetres.
Conversion table
| Physical size | 72 DPI | 150 DPI | 200 DPI | 300 DPI |
|---|---|---|---|---|
| 3.5 x 4.5 cm (photo) | 99 x 128 | 207 x 266 | 276 x 354 | 413 x 531 |
| 2 x 2 inch (square photo) | 144 x 144 | 300 x 300 | 400 x 400 | 600 x 600 |
| 4 x 2 cm (signature) | 113 x 57 | 236 x 118 | 315 x 157 | 472 x 236 |
| 4 x 6 inch (photo print) | 288 x 432 | 600 x 900 | 800 x 1200 | 1200 x 1800 |
| A4 (21 x 29.7 cm) | 595 x 842 | 1240 x 1754 | 1654 x 2339 | 2480 x 3508 |
Why 350 x 450 pixels is called 3.5 x 4.5 cm
Many portals pair "3.5 x 4.5 cm" with "350 x 450 pixels". Check the arithmetic: 350 pixels across 3.5 cm is 100 pixels per centimetre, which is 100 x 2.54 = 254 DPI. So that pairing assumes about 254 DPI. At 300 DPI the same 350 x 450 file would print at only about 2.96 x 3.81 cm.
The practical rule: when a form gives both a physical size and a pixel size, the pixel size is what the upload check measures. The centimetre figure is a description of how it is meant to look when printed. If they disagree, follow the pixels.
Does the DPI number inside the file matter?
Almost never for an online upload. A JPEG can carry a DPI tag (72, 96, 300), but a portal that checks dimensions looks at the pixel width and height, not the tag. Changing only the DPI tag without changing the pixel count does not change how the picture looks on screen and will not make a file pass a pixel check.
DPI matters when you print. A photo meant for a printed 4 x 6 inch card needs about 1200 x 1800 pixels (300 DPI) to look sharp. That is why the ApplyReady passport photo print sheet builds a 300 DPI sheet.
Common mistakes
- Using the wrong divisor. There are 2.54 centimetres in an inch, not 2.5 and not 10.
- Mixing millimetres and centimetres. 35 x 45 mm is the same as 3.5 x 4.5 cm.
- Stretching to fit. If your crop has a different ratio from the target, crop first. Setting width and height independently squashes the face.
- Assuming bigger is safer. Many portals have a maximum pixel size as well as a minimum. Stay inside both.
In the ApplyReady resizer you can switch the unit selector between pixels, centimetres and inches and set a DPI directly, and the tool does the conversion for you. Understanding the formula lets you check the answer. For the file-size side of the problem, read pixels or kilobytes.
Frequently asked questions
Does changing the DPI make my file smaller?
Not by itself. File size depends on the number of pixels and the compression, not on the DPI tag. Changing only the DPI tag leaves the pixel count, and the file size, almost the same.
A form asks for 200 x 230 pixels. What is that in centimetres?
There is no single answer, because it depends on DPI. At 100 DPI it is about 5.1 x 5.8 cm; at 300 DPI it is about 1.7 x 2.0 cm. When a form gives pixels, use pixels.
What DPI should I use for printing a photo?
300 DPI is the usual target for a sharp print. For a 4 x 6 inch print that means 1200 x 1800 pixels. Lower values such as 150 DPI are acceptable for casual prints but look softer up close.
Why do two tools show different sizes for the same file?
One may display the pixel size, another the size at the file's DPI tag. The same photo can be listed as 3000 x 4000 pixels or as "25 x 33 cm at 300 DPI". Always compare pixel dimensions with pixel dimensions.
What is the difference between DPI and PPI?
PPI (pixels per inch) describes a screen or an image's pixel density, and DPI (dots per inch) describes a printer's ink dots. In everyday use, the two terms are used interchangeably for photo sizing.